Project Summary One of the most important challenges facing biology today is making sense of genetic variation. Un- derstanding how genotypic variation translates into phenotypic variation and how it is structured in populations is fundamental to our understanding of evolution, and has enormous practical implications for human health as well as for agriculture and conservation. The long-term objective of this project is to increase our understanding of the molecular genetic basis for adaptive variation by studying flower- ing time in A. thaliana. The focus is on the flowering response to cold temperatures, so-called vernal- ization, which is one of the major mechanisms plants use to ensure that they flower at the right time, during the right season. The project seeks to describe the genetic architecture underlying variation for this trait, and will identify, at the molecular level, the major genes and alleles involved. The adaptive significance of the identified polymorphisms will be determined in field trials. The project has three spe- cific aims: First, to map the genes responsible for variation between plants collected in different parts of the world. The mapping will be done using a combination of traditional linkage mapping methods and so-called genome-wide association scans, in which modern genotyping technology is used to survey massive amounts polymorphisms in population samples in order to identify genomic regions that appear to be statistically associated with the trait. Second, molecular genetics will be used to characterize the identified alleles and loci. Third, the pattern of variation in and around identified loci will be analyzed in order to elucidate the history of selection on the loci. Project Narrative One of the most important challenges facing biology today is understanding how genetic variation be- tween individuals translates into variation we can see or measure, like blood pressure in humans, or drought tolerance in rice. The goal of this project is to increase our understand of the general principles that underlie the genetics of adaptive natural variation by studying flowering time in the model plant thale cress (Arabidopsis thaliana). The focus is on the flowering response to cold temperatures, so-called vernalization, which is one of the major mechanisms plants use to ensure that they flower at the right time, during the right season.